Internal gear pumps are used as, for example, pumps for lubricating engines and automatic transmissions (AT) in vehicles. One known type of such an internal gear pump is formed by combining an inner rotor and an outer rotor, between which a difference in the number of teeth is one, and disposing the rotors eccentrically relative to each other. Furthermore, in another known pump of this type, the tooth profile of the rotors is formed by using a trochoidal curve, which is known for good volume efficiency, low noise, and low drive torque.
A tooth profile formed by using this trochoidal curve is formed in the following manner. First, as shown in FIG. 5, a rolling circle B rolls along a base circle A without slipping, and a trochoidal curve TC is drawn by a locus of a point on a radius distant from the center of the rolling circle B by a distance e (=amount of eccentricity between rotation centers of the inner rotor and the outer rotor). Then, the tooth profile of the inner rotor 2 is formed by an envelope of a group of circular arcs of a locus circle C having a fixed diameter and whose center is located on the trochoidal curve TC (also see Patent Literature 1 below).
In a pump having a tooth profile using such a trochoidal curve, an amount E of eccentricity between the center of the inner rotor and the center of the outer rotor is regulated for ensuring the face width and for designing the tooth profile. Therefore, an increase in the tooth height is limited, making it difficult to fulfill demands for increasing the discharge rate. The present applicant has made a proposition in Patent Literature 2 below in which the tooth height can be freely set in a pump rotor of the aforementioned type.